I mean they already charge 1-3% why not 30% if they can get away with it? What are you going to do, not accept credit cards? Take cash on your website? I think people are rightfully mad when they realize how much leverage someone has over them but it doesn't usually change that fact.
If there are alternatives, people will use them. Starbucks created a whole payment/reward system to avoid credit card fees, for example. If there were cheaper alternatives possible to the app stores, you'd better be sure some developers would use them.
